The North had to build an army from scratch. The regular army at the start of the war was about 16,000 troops. Half of that force along with its best officers went south to join the Confederacy. There were a lot of political appointees getting generalships. They had to be weeded out over time/failures.

McClellan built the Army of the Potomac into a real fighting force....but he didn't know how to lead it in combat. Until Grant and Sherman advanced up the ladder into their positions the North was at a disadvantage in leadership. Then you have to consider while all this was going on that they were trying to subdue a very bitter and determined enemy that was fighting on its homeground. The war couldn't be fought any other way in subduing the southern states.